An aircraft's gas turbine engine is typically equipped with a bearing lubrication system that lubricates bearings supporting rotating shafts associated with the engine. One known bearing lubrication system continually directs a lubricant, such as oil, over the bearings utilizing a spray bar mounted in the bearing housing. After flowing over the bearings, the oil collects in a sump provided at the bottom of the bearing housing. The oil may then flow from the sump into an oil tank, which holds a large reservoir of oil. Under the influence of a supply pump, the oil may then be drawn from the oil tank and supplied to the spray bar, which again directs the oil over the bearings. This cycle is continually repeated to maintain the bearings at a proper lubrication.
An air cavity exists within the bearing housing above the sump. During flight, this air cavity may become pressurized due to leakage across the carbon seals of the gas turbine engine. To relieve this pressure, a conventional lubrication system may vent the air cavity (and, perhaps, the oil tank) to atmospheric pressure. As a result of this leakage and venting, the pressure within the air cavity fluctuates as a function of aircraft flight altitude. Thus, at high flight altitudes (e.g., 60,000 feet above sea level), the pressure within the air cavity may become extremely low; and, at low flight altitudes (e.g., 1,000 feet below sea level), the pressure within the air cavity may become relatively high. When the pressure within the air cavity becomes too low, pump cavitation may occur (i.e., the sudden formation and collapse of low pressure bubbles). Cavitation may increase wear on the supply pump and the other lubrication system components. In addition, cavitation may interrupt the continuity of oil flow and thus result in a non-uniform spray over the bearing. Conversely, when the pressure within the air cavity becomes too high, oil may leak from the lubrication system.
It is thus desirable to provide an overboard vent valve, and an aircraft bearing lubrication system employing such a vent valve, which maintains the pressure of the bearing housing's air cavity above a minimum pressure threshold to prevent pump cavitation. It would also be desirable if such an overboard vent valve also maintained the pressure of the bearing housing air cavity below a maximum pressure threshold to minimize or eliminate oil leakage. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.